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MotoIQ Project Subaru WRX STi

Project Subaru WRX STi Part 3: Streetable Track Worthy Suspension

By Mike Kojima

In our last segment of Project Subaru WRX STi we tackled the car’s weakest area, its handling, and made significant improvements by installing Whiteline’s basic handling pack suspension kit. Although we were very favorably impressed with the kit’s enhancement of Project STi, we needed more. We wanted more than just a better handling car; we wanted one that had the potential to put the hurt on the competition in time attack events as well as a decent street ride because this car also serves as a daily driver.

To see the other articles in this series, click here!

Although the stock STi dampers are valved more stiffly than the base model WRX, we wanted something with higher spring rates and more damping, especially with our sticky tires. When pushed hard our car would exhibit float, body roll and poor transient response. The STi had an edgy feel to it where it would break loose when turning in without giving much warning to the driver.

KW Varient III suspension
The KW Variant III suspension is one of the most advanced street suspension packages on the market featuring independently adjustable compression and rebound damping, remote accumulators and sophisticated multistage frequency sensitive valving.

We also wanted to lower our ride height to lower our center of gravity and to reduce weight transfer under cornering to the outside tires without reducing bump travel. What most people don’t realize is that simply lowering many cars reduces bump travel to the point where the car rolls onto the bump stops as soon as hard cornering is established. This causes severe over or understeer depending on which end of the car runs out of travel first. In order to handle well the car must be kept off the bump stops. This is the first golden rule of good handling that most people ignore.


The first and most critical parts of any suspension build are the dampers and springs.  We chose KW Suspensions Variant III coilovers because we feel that they are one of the best street suspensions out there for the serious driver. Variant III’s are a unique advanced gas charged double damping adjustable twin tube damper.  Although some people may dismiss twin tubes as low end dampers compared to monotubes, the KW’s are a high precision twin tube with the characteristics of a monotube.  The KW Variant III’s independent adjustability of compression and rebound damping is also a huge advantage over other streetable suspensions. Independent adjustability of compression and rebound damping is important for any car that is going to be tuned for optimal handling under a wide range of set ups and conditions like a dual purpose street/track car.

KW Variant III rebound fluid flow
The low speed rebound damping is controlled mostly by the piston on the end of the shock rod.  Turning the rebound adjusting knob located on the top of the shaft screws the tapered metering rod "V" up and down varying the size of the orifice.  The low speed fluid flow is represented by the green arrows.  At higher shaft speeds the mid and high speed circuit kicks in, the high speed fluid flow is shown by the blue arrows. The hydraulic pressure deflects the metering disc shown in yellow for mid speed damping and higher velocities allow the spring loaded blow off of the disc to open.

The rebound adjustment primarily affects the low speed damping--shock piston velocities in the 0-2" per second range.  Low speed damping adjustment affects body motions such as roll and pitch.  This is in the area which the driver feels the most when driving fast.  The V3 has the main shaft and piston riding in the inner tube of the shock body with the rebound damping adjuster built into the low speed circuit.  A threaded tapered rod that goes to a knob at the top of the shaft controls the size of the low speed damping orifice.  The knob is adjusted with an allen wrench. Screwing it in and out raises and lowers the tapered end of the rod into the orifice which changes the amount of fluid that can flow through it.  When the orifice is mostly blocked by the taper, little oil can flow and there is a lot of low speed damping. When the orifice is open, the fluid can flow freely and the damping is decreased. 

The piston also has a unique two stage system combining a deflected disc valve system and a spring loaded blow off valve for controlling the rest of the velocity range.
The foot valve lives on the bottom of the inner tube and regulates the flow of fluid between the inner and outer tubes. A spring loaded needle valve controls the size of the low speed compression damping orifice located in the foot valve as well as the valve spring preload.  The needle valve is adjusted via an allen screw on the bottom of the shock body.  Like the rebound, the compression adjuster also mostly influences low speed compression damping, which like rebound is the part of the damping curve that influences body motion.  On rebound the fluid flows through orifices and the damping force is controlled by the unique combination of deflected dics valves with a spring loaded blowoff function. The combination of deflected disc valving and spring loaded blow off give KW’s their unique range of control with frequency sensitivity.  This helps the shocks produce both firm control and a smooth non tire shocking ride.

KW variant 3 foot valve fluid flow
In rebound, the shocks foot valve located on the bottom of the shock, simply acts like a one way check valve allowing fluid displaced by the shock shaft to flow into the outer tube and the hydraulic accumulator.

 

In grassroots motorsports where you don’t have a large pit crew and in dual purpose track street cars, being able to quickly and easily adjust the damping is important.  Some double adjustable shocks require disassembly of the shock assembly so a button can be pushed while the shaft is rotated and other time consuming methods.  The KW’s easy to reach external adjustments make dialing in the chassis a lot easier.

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Comments
Fly'n_Z
Fly'n_Zlink
Monday, March 15, 2010 9:42 PM
Sweet article guys, one of the best yet! I know that you're California based but do you think that this suspension could be 4 season capable?

In the winter I would mount a set of snow tires on the stock wheels of course but could I otherwise leave things in place or would I be better off mounting the OEM shocks/springs/bars for a more benign winter handling balance?
Mike Kojima
Mike Kojimalink
Tuesday, March 16, 2010 1:01 AM
The V3's have more anti corrosion stuff than any coil over on the market so I would say they would work in winter.
urbanjacup
urbanjacuplink
Tuesday, March 16, 2010 2:17 AM
"Even though I have driven and fallen in love with several modified EVO’s in various track events, I was surprised to find myself liking Project STi better."
(very encouraging...)

Looking forward to EVO vs STI...
BenFenner
BenFennerlink
Tuesday, March 16, 2010 4:00 AM
Great article! I learned a lot and won't keep it to myself.
Dave Coleman
Dave Colemanlink
Tuesday, March 16, 2010 4:50 AM
How much snow driving have you done, Mike? I'd normally be concerned about a stiff suspension in the snow if it were anything but a Subaru. On snow tires, with all that subaruness its probably still passable in the winter, but far from ideal.

I haven't driven this car, so take my opinion with a grain of salt. I don't think Mike has ever seen snow, though, so take his with one too...
Mike Kojima
Mike Kojimalink
Tuesday, March 16, 2010 5:37 AM
Of course the suspension would not be snow optimized but neither is the stock one. I was talking about from a corrosion standpoint. East Coast snow driven cars crumble with corrosion. Mot coil overs would fall apart in one good winter.

If you wanted to be more snow optimized, then disconnect the bars and raise the ride height and soften up the shocks. Yes I have driven in snow.
MisteenoMike
MisteenoMikelink
Tuesday, March 16, 2010 7:22 AM
Are these KW's independently height adjustable, without having to move the spring perch? (I'm of the impression they're not). Read nothing but good things about these units everywhere but was just curious why they felt that feature was not necessary. Thoughts?
Mike Kojima
Mike Kojimalink
Tuesday, March 16, 2010 8:39 AM
No they are not, KW's design philosophy feels that that feature would encourage overlowering, which would mess up the suspension geometry.
spdracerut
spdracerutlink
Tuesday, March 16, 2010 8:45 AM
Very interesting about the crossmember lock. I bet that went a long way in making the handling more predictable. Having owned an Evo, and tracked a STI once, those steering rack bushings for the STI are absolutely necessary! The stock bushings had gotten so torn up in the car I tracked, the steering wheel was shaking a good half-inch in all sorts of directions! Very un-nerving....

And the diff clunk on WRXs is annoying. Glad there are bushings for that.
JustTheTip
JustTheTiplink
Tuesday, March 16, 2010 12:46 PM
Is there a reason that I see a huge XS front mount in some of the pictures and not in others?

Otherwise good job!
Mike Kojima
Mike Kojimalink
Tuesday, March 16, 2010 1:03 PM
Good Eye! Some of the pictures were taken at different times.
Steve
Stevelink
Tuesday, March 16, 2010 6:14 PM
@Dave - as of course you know, almost anything *can* be driven in snow, but I only have spring rates in the 400 range and softer OEM Nissan dampers:

http://picasaweb.google.com/lh/photo/3hXn0RGWkTeTPyyoKAEWQw?feat=directlink

But sure as hell fun! Team O'Reilly Rally School up in NH, nice place.
http://picasaweb.google.com/skylinegtr01/AudiClubWinterDriving#

But your old B13 is a good example of a car that would go nowhere in snow w/o a lot of encouragement, even if I drove it in Winter. Upped the springs with new set of Progress CO's last Aug to 450/350 and major handling improvement, but even more useless in snow.

Nice article, great writeup on the variants.
Steve
Stevelink
Tuesday, March 16, 2010 6:18 PM
@Mike - IDK if that's really true about a season or two. My STi friends drive their cars mostly year 'round in New England, and corrosion of the body is a much bigger concern than corrosion of the CO's. I also use Fluid Film on all the exposed stuff, a sort of parrafin based spray with who knows what in it. Plow drivers swear by it and salt spray won't stick to it, my truck is much happier now. GREAT stuff. Also sprayed down all the suspension and steel bits on my G, which sees a good amount of winter use.
Steve
Stevelink
Tuesday, March 16, 2010 6:19 PM
Man how do I change this?
http://www.gravatar.com/avatar/5bd149964e5e6b0dd6535a4fbf567630?d=wavatar
Mike Kojima
Mike Kojimalink
Wednesday, March 17, 2010 6:09 AM
Go to gravatar.com and sign up using the same email address u used to register for motoiq...
Eric Hsu
Eric Hsulink
Wednesday, March 17, 2010 2:43 PM
Does Aaron still have this car? That FMIC rocks dude hahah.
mikemiessler
mikemiesslerlink
Wednesday, March 17, 2010 6:52 PM
Granted this is a Subaru and it was engineered to haul pony-dick through the woods and pull stumps out of gay farmer's feilds but for snow driving wouldn't it be better to run a softer suspension and focus more on steering feedback? There's a guy on our forum (www.ga16forum.com) that ice races a B12 awd Sentra wagon with a turbo'd GA who from his pictures doesn't have anything crazy in terms of suspension. He seems to just use steering feedback and sliding to navigate turns and scrub of speed respectively. Im thinking the STI setup or maybe just a decent spring/shock combo would be a better way to go than full coilovers for the snow. Sry to nitpick at your weathered-ness. :D
Mike Kojima
Mike Kojimalink
Wednesday, March 17, 2010 10:43 PM
If you get down to it, really soft suspension and really skinny tires work the best for snow, look at the WRC cars when they run in snow, but the question was could this stuff be run year round without damaging it and the answer is yes.

Is this what I would run for ice racing? Maybe, I have never iced raced but I could ask a few people.

I have never raced in snow but I do real well in the rain and all I do is soften the shocks and lower my air pressure.
KW
KWlink
Thursday, April 01, 2010 8:16 AM
Due to its German roots, KW originally designed their coilovers to function in the varying weather conditions all over Europe. The oil inside the coilovers is actually a very costly aviation grade fluid that is designed to maintain viscosity and thus damping consistency down to -40F, temperatures you would experience at high altitudes.

Cold weather conditions would not degrade the damping forces or consistency even if you parked your car out in the snow.

As for ice racing applications, while there is some adjustability in the low speed compression and rebound forces to help the tires dig in, the high speed profiles would be too agressive. If you are looking for an ice racing application, KW does custom build competition applications specific to your vehicle and needs. This means spring, damping forces, knee points, and even stroke can be changed.

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